Balanced tri-band bandpass filter using sext-mode stepped-impedance square ring loaded resonators

: In this letter, a novel balanced tri-band bandpass fi lter (BPF) is developed by using the proposed sext-mode stepped-impedance square ring loaded resonators (SI-SRLR). The new SI-SRLR has six resonant modes, including three di ff erential-modes (DMs) and three common modes (CMs). The operating mechanism of these resonances are explored by the even- / odd-mode method and simulation techniques. Three DMs of the proposed resonator are used to build the triple DM passbands. The appropriately admittance ratio K of SI-SRLR is chosen to inhibit the CM signal in within three DM passbands. Moreover, three T-shaped stubs are loaded to enhance the CM suppression. Finally, a balanced tri-band BPF with compact size and high selectivity is designed, fabricated, and measured. The measurements are in good agreement with the simulated results, which veri fi es well the proposed structure and design method.